Aluminum nitride does not have a melting point under normal pressure and is decomposed at a temperature of 2,500.degree. C. or higher. Aluminum nitride is therefore used as a sintered body except single-crystal thin films.
The mechanical strength of an aluminum nitride (AlN) sintered body is rarely degraded at high temperatures, and its durability against some chemicals is excellent. The aluminum nitride sintered body is used as a heat-resistive material due to the above properties. The aluminum nitride sintered body is also promising as a radiating material of a semiconductor device due to its high thermal conductivity, and a circuit board insulating material due to its good electric insulation properties
An aluminum nitride sintered body is normally obtained by molding and firing an aluminum nitride powder. When a very fine AlN powder (particle size: 0.3 .mu.m or less) is used, a substantially full-densed AlN sintered body can be obtained without using a sintering additive. However, since oxygen in an oxide layer on the surface of the raw powder consolidates into the AlN lattice during sintering or transforms to Al--O--N compounds, the maximum thermal conductivity of the sintered body without using any sintering additive is about 100 W/m.multidot.K. When an AlN powder (particle size: 0.5 .mu.m or more) is used, the sintering properties are not so good. It is therefore difficult to obtain a full-densed sintered body without adding a sintering additive in a method excluding hot pressing In order to obtain an AlN sintered body at normal pressure, a rare earth metal oxide or an alkali earth metal oxide is added as a sintering additive to increase the density of the sintered body and prevent impurity oxygen in the surface layer of the AlN material powder from consolidation (Japanese Patent Disclosure Nos. 60-127267, 61-10071, and 60-71575). Since such a sintering additive is reacted with impurity oxide in the surface layer of the AlN raw powder to promotes a liquid phase sintering, the density of the sintered body can be increased. Also, impurity oxygen is fixed (oxygen trapping) as grain boundary phases outside the AlN grains, thereby a high thermal conductivity can be performed.
When a sintering additive is added to the AlN raw powder, the density and thermal conductivity of the resultant AlN sintered body are improved. However, the thermal conductivity is only a maximum of about 150 W/m.multidot.K due to grain boundary oxide phases (i.e., second phases with respect to the AlN phase as the main phase) produced as a result of addition of the sintering additive and the incompletely trapped oxygen atoms. This thermal conductivity is much lower than AlN's theoretical thermal conductivity of 320 W/m.multidot.K.
Various attemps have been made to improve the thermal conductivity of an aluminum nitride sintered body. But not satisfactory results have been obtained.